fix(frontend): replace useState with useRef for auto-save timeout in DocumentEditor

[hemish22]

Summary

Fixes #325

• saveTimeout was useState → every setSaveTimeout call triggered useEffect re-run → infinite debounce loop (new timeout created before old one fires)
• Replaced with useRef — stores timer ID without causing re-renders
• Removed redundant setIsSaving(true/false) in timeout callback (handleSave already manages this)

Changes

File	Change
frontend/src/components/DocumentEditor.tsx	useState → useRef for timeout; remove redundant state calls; saveTimeout removed from effect deps

Before / After

Before — effect deps included saveTimeout:

const [saveTimeout, setSaveTimeout] = useState<...>(null)
// ...
setSaveTimeout(timeout)          // ← triggers effect re-run → infinite loop
// ...
}, [content, handleSave, initialContent, saveTimeout])

After — ref never triggers re-render:

const saveTimeoutRef = useRef<...>(null)
// ...
saveTimeoutRef.current = setTimeout(...)   // ← no re-render
// ...
}, [content, handleSave, initialContent])

Test Plan

• Edit document → auto-save fires once after 2s (not repeatedly)
• Disconnect backend → save error banner appears in toolbar
• Manual Save button → shows "Saving…" then error on failure
• No console errors on mount or typing

AI Disclosure

Fix implemented with Claude Code (claude-sonnet-4-6).